Process for the production of aromatic polycarboxylic acids



Unite PROCESS FOR THE PRODUCTION OF AROMATIC This invention relates to a process for the production of aromatic or aromatic heterocyclic diand tricarboxylic acids from aromatic or aromatic heterocyclic monocarboxylic acids.

As the applicants had previously found, the alkali metal salts of carboxylic acids, the carboxyl groups of which are attached to aromatic ring systems or to heterocyclic rings having an aromatic structure, can be transformed into salts of other aromatic carboxylic acids with at least two carboxyl groups in the molecule, by heating to elevated temperatures. When salts of monocarboxylic acids are used as starting materials, the salts of diand tricarboxylic acids are obtained as reaction products. The industrially valuable reaction products formed thereby are, for example, terephthalic acid, trimesic acid, naphthalene-2,6-dicarboxylic acid, 4,4'-diphenyldicarboxylic acid, pyridine-2,5-dicarboxylic acid, pyridine-2,4,6-tricarboxylic acid, furan-2,5-dicarboxylic acid, thiophene- 2,5 dicarboxylic acid and many others. The ring systems free from carboxyl groups are obtained as byproducts. These processes were carried out with or without pressure in the presence of an inert protective gas, preferably carbon dioxide. When pressure has been used heretofore, the reaction has been carried out at pressures up to about 250 atmospheres.

It is an object of this invention to produce desired aromatic carboxylic acids in increased yield in an aromatic carboxylic acid conversion process conducted at carbon dioxide pressures in excess of 400 atmospheres.

Another object is to avoid the formation of undesirable side products in the production of aromatic or heterocyclic diand tricarboxylic acids from the corresponding monocarboxylic acids.

A further object of the present invention is a process for the production of aromatic or aromatic heterocyclic diand tricarboxylic acids from the corresponding monocarboxylic acids which is carried out at lower temperatures than heretofore.

These and other objects will become apparent as the description of this invention proceeds.

We have found that the transformation of salts of aromatic monocarboxylic acids or of heterocyclic monocarboxylic acids having an aromatic structure into salts of dior tricarboxylic acids may be carried out especially advantageously by heating the starting materials in the presence of acid binding agents to elevated temperatures under carbon dioxide pressure and by working under pressures above 400 atmospheres.

The starting materials for the process according to this invention are salts of aromatic monocarboxylic acids. Such acids are, for example, benzoic acid, aand 5- naphthoic acid, diphenylmonocarboxylic acids. Also, monocarboxylic acids in which the carboxyl groups are attached to another aromatic ring system such as to anthracene, terphenyl, diphenylmethane or benzophenone radicals, are suitable as starting materials for the process in accordance with the invention.

Similarly, the starting materials for the process according to the invention may be salts of monobasic heterocyclic carboxylic acids, the carboxyl groups of which are attached to heterocyclic rings having an aromatic struc- States Patent ture. Such acids are derived, for example, from pyridine, pyrazine, pyrimidine, pyridazine, a-pyran, furan, thiophene, thiazole,- quinoline, isoquinoline, indole, benzotriazole 0nd benzimidazole.

In all of these carboxylic acids the aromatic ring or the heterocyclic ring having an aromatic structure may in addition to the carboxyl group also carry other substituents such as halogen atoms or alkyl radicals, provided that they do not decompose at temperatures below the reaction temperature. The term aromatic carboxylic acids is, therefore, intended to include both compounds having a homocyclic aromatic ring and compounds having a heterocyclic ring.

The above-named carboxylic acids are used in the form of their salts for the process according to this invention. Advantageously, the alkali metal salts are used, and preferably the potassium salts and in addition also the sodium salts. The lithium, rubidium and cesium salts which may also be employed must generally be excluded because of economic reasons. It is often advantageous to use mixtures of salts of two different metals, for example, mixtures of the sodium and potassium salts, because in many cases the mechanical properties of the reaction material are improved thereby.

In place of such salts, reaction materials which form the salts may be used. Particularly suitable materials are carboxylic acid anhydrides or also carboxylic acid esters and acid-binding metal compounds, such as alkali metal carbonates. These mixtures do not need to be provided in stoichiometric ratios; One or the other component may be used in excess.

It is advantageous to carry out the reaction according to this invention in the presence of acid-binding agents, preferably in the presence of alkali metal carbonates, alkali metal formates or alkali metal oxalates. The abovementioned acid 'binding agents do not need to be employed in stoichiometric quantities. They may be pro- Vided in quantities less than the stoichiometric amount or also in excess.

The salts or salt mixtures to be subjected to the reaction are preferably provided in as dry a condition as possible. If the salts are available in the form of their aqueous solutions they may be transformed into dry powders in accordance With known methods, preferably by spray-drying, and if necessary, subjected to a subsequent drying treatment to remove minute residual quantities of moisture.

It has further been found that the reaction according to the present invention is favorably influenced by the presence of catalysts. Metals such as zinc, cadmium, mercury, lead and iron, as Well as compounds of these metals, such as their oxides, or their inorganic or organic acid salts, for example, their carbonates, bicarbonates, halides, sulfates, phosphates, acetates, formates, oxalates, fatty acid salts or also the salts of the above-mentioned metals formed from those acids which are employed as starting materials for the reaction according to the invention or which are formed during the reaction, for example, their benzoates, phthalates or terephthalates, may be used as catalysts. The amount of catalyst may vary Within wide limits and may range from 0 to 15% by weight, preferably from 0.5 to 5% by weight, based on the weight of reaction mixture. The catalyst may be uniformly and finely distributed throughout the reaction mixture by spray-drying or otherwise transforming an aqueous solution of the salts serving as the starting material, which has the catalyst dissolved or suspended therein, into a dry powder. The above-named catalysts may also be employed in conjunction with known carrier, for example, with kieselguhr.

The reaction according to the present invention may not only be carried out in the presence of these catalysts aoaaeae but also in the presence of liquid or solid additives, for example, in the presence of sand, metal powder, metal shavings, kieselguhr, activated charcoal, finely divided aluminum oxide, finely divided silicic acid, or also, inert salts such as sodium sulfate. In many cases the mechanical properties of the reaction mixture are improved by these additives. In place of the solid inert materials, inert liquids which do not decompose under the prevailing reaction conditions may also be used, such as toluene, benzene or the like.

The high pressure required for the reaction which exceeds 400 atmospheres, and preferably more than 500 atmospheres, may be produced in a very simple fashion, for example, by suitable pumps or compressors. The high pressures may, however, also be produced by passing liquid carbon dioxide from a pressure cylinder or another storage vessel into the cooled andevacuated reaction vessel, and thereafter heating the same. In place of liquid carbon dioxide, solid carbon dioxide may also be used. Pressures of 1500 to 2000 atmospheres 'are developed thereby due to the temperature required for the reaction, depending upon the amount of carbon dioxide introduced into the reaction vessel. Otherwise there is no upper pressure limit but the upper limit depends largely upon the strength of the available apparatus. As a rule, the reaction begins at temperatures between 300 and 400 C. The optimum reaction temperature is different and. depends upon the starting materials used. Sometimes it is advantageous toemploy a reaction temperature below 400 C., but the upper temperature limit for the process is determined only by the decomposition temperature of the organic starting materials and reaction prevent the reaction mixture from sintering or'caking.

This may, for example, be accomplished by performing the reaction in vessels. provided with a stirring device, in'roeking autoclaves or in rotary autoclaves. Uniform heating of the reaction material may alsobe eifected by distributing thereaction material ,in thin layers with or without agitation.

However, good 'yields are also obtained without applying these particular measures, provided care is taken that strong local overheating is avoided.

The separation of the reaction product from the reaction material may take place in known fashion. The raw product is first dissolved in water or in dilute acids and thereafter purified by filtration or by treatment with activated charcoal or with other decoloring agents, if necessary. Subsequently the salts formed by the reaction may be transformed into the corresponding free acids by acidification with organic or inorganic acids or also by passing carbon dioxide therethrough with or without pressure. Thefree acids may be separated by making use of their different 'solubilities or volatilities, and may thereafter be isolated in relatively pure form and, if desired, transformed into their derivatives. The

:salt mixtures produced by the reaction may also be transformed directly into derivatives of the acids, for example, into their esters or halides, and these derivatives may then be purified by fractional distillation, if desired.

The process in accordance with this invention produces industrially valuable di-- and polycarboxylic acids or their salts or derivatives, such as terephthalic acid, trimesic acid, naphthalene-2,6-dicarboxylic acid, pyridine-2,5-dicarboxylic acid, pyridine-2,4,6-tricarboxylic acid, furan- 2,5-dicarboxylic acid, thiophene-2,5-dicarboxylic acid, and many others.

The advantage of the process according to the present invention over the methods heretofore used in which pressures upto a maximum of 250.atmospheres were employed, resides in that the present'procedure produces a substantial improvement in the yield and the amount of by-product consisting of the ring system free from carboxyl groups is reduced and even completely eliminated. A further substantial advantage of carrying out the reaction at pressures above '400 atmospheres in accordance with the present invention is that in many cases the optimum reaction temperature is considerably reduced. Furthermore, the process according to the present invention permits the production with good yields also of those reaction products which were not accessible at all or only in minute quantities by the procedures heretofore used.

The following examples are set forth to enable persons skilled in the art to understand and practice our invention but we do not intend to be limited thereby.

Example I 30 gm. potassium benzoate, 13 gm. anhydrous potassium carbonate (molar ratio 1:05) and 1 gm. cadmium fluoride were milled in a ball mil-l and the mixture was placed into an autoclave having a net volume of 0.2 liter. About gm. liquid carbon dioxide were then introduced, and the contents of the autoclave were heated for 7 hours at 360 C. whereby a maximum pressure of 1540 atmospheres developed. The reaction temperature was measured in this and the following Examples II-XI by means of a thermoelectric couple which was in the center of the reaction chamber. According to'experience the wall temperature lies about 20-50 C. higher thanJthe measured temperature.

The reaction product was dissolved in water and the terephthalic acid formed by the reaction was precipitated with hydrochloric acid. 20.7 gm. terephthalic acid were obtained which was pure. From the mother liquors 1.3% of the quantity of benzoic acid originally used were recovered. Taking into consideration the amount of recovered benzoic acid, the yield of terephthalic acid was 65.6% of theory. The calculation of the yield was made under the assumption that 1 mol benzoic acid forms 1 mol terephthalic acid.

Example II 30 gm. potassium benzoate, 13 gm. anhydrous potassium carbonate, '1 gm. cadmium fluoride and about 150 gm. liquid carbon dioxide were heated for 7 hours at 380 C. in the same manner as described in Example I whereby a maximum pressure of about 1600 atmospheres was reached. After further processing of the reaction mixture, no benzoic acid was recovered. The yield of terephthalic acid was 61.5%, calculated on the assumptiog that 1 mol benzoic acid forms 1 mol terephthalic aci Example III 30 gm. potassium benzoate, 13 :gm. anhydrous potassiumcarbonate, 1 gm. cadmium fluoride and about 150 gm. liquid carbon dioxide were heated for 7"hours at 350 C. under the same conditions as those described in the two preceding examples, whereby a maximum pressure of 1300 atmospheres was reached. Taking into consideration the 4.4 gm. 'of recovered benzoic acid, a terephthalic acid yield of 65.6% was obtained.

Example IV 43 gm. of a mixture consisting of'potassium benzoate and potassium carbonate in a molar ratio of 12-1 (corresponding to 23.1 gm. potassium benzoate) which was produced by simultaneously spray-drying corresponding solutions, were admixed with 2 gm. cadmium fluoride in a ball mill and the resulting mixture was placed into an autoclave having a net volume of 0.2 liter. About 150 gm. liquid carbon dioxide were added thereto. Subsequently, the contents of the autoclave were heated for 7 hours at 360 C. whereby a maximum pressure of about 1000 atmospheres was reached. Upon working up the reaction mixture in the same manner as described in Example I, 34% of the amount of benzoic acid originally used were recovered. The yield of terephthalic acid was 11.45 gm., which corresponds to 72.8% of the theoretical yield.

Example V 43 gm. of an equimolar mixture of potassium benzoate and potassium carbonate and about 150 gm. liquid carbon dioxide were heated for 21 hours at 330 C. under the same conditions as those described in Example IV. A maximum pressure of about 1100 atmospheres was reached. After further processing of the reaction mixture, 3 gm. benzoic acid were recovered. The yield of terephthalic acid was 15.6 gm. which corresponds to.79% of the theoretical vield.

Example VI A homogeneous mixture of 30 gm. potassium benzoate, 13 gm., anhydrous potassium carbonate and 2 gm. of cadmium fluoride was placed into an autoclave having a net volume of 0.2 liter. About 160 gm. liquid carbon dioxide were added thereto. The contents of the autoclave were then heated for 7 hours at 360 0, whereby a pressure of about 1600 atmospheres was reached. The reaction mixture was further processed in the manner described in Example I. 0.3 gm. benzoic acid was recovered from the mother liquid. The yield of terepht halic acid was 20.7 gm. Taking into consideration the amount of benzoic acid recovered, this corresponds to a yield of 69.9% of theory.

Example VII A homogeneous mixture of 30 gm. potassium benzoate and 13 gm. anhydrous potassium carbonate was placed into an autoclave having a net volume of 0.2 liter. About 160 gm. liquid carbon dioxide were added thereto. Thereafter the contents of the autoclave were heated for 7 hours at 360 (3., whereby a pressure of about 1600 atmospheres was reached. After further processing of the reaction product in the same manner as described in Example I, 4.95 gm. benzoic acid were recovered. The yield of terephthalic acid was 9 gm. which corresponds to a yield of 52.8% of theory, taking into consideration the amount of benzoic acid recovered.

Example VIII 40 gm. potassium benzoate and about 160 gm. liquid carbon dioxide were placed into an autoclave having a net volume of 0.2 liter. Subsequently, the contents of the autoclave were heated for 7 hours at 360 C. whereby a pressure of about 1600 atmospheres was reached. After treatment of the reaction mixture in the samemanner as described in Example I, 18.55 gm. benzoic acid. were recovered. The yield of terephthalic acid was-6.1 gm, which corresponds to a yield of 37.4% of theory when taking into consideration the amount of benzoic acid recovered. 4

Example IX A homogeneous mixture of 30 gm. potassium benzoate, 13 gm. anhydrous potassium carbonate and 2 gm. cadmium fluoride was placed into an autoclave having a net volume of 0.2 liter. About 40 gm. liquid carbon dioxide were added thereto. Thereafter the contents of the autoclave were heated for 7 hours at 380 C. whereby a pressure of 500 atmospheres was reached. The reaction mixture was worked up in the same manner as described in Example 1. From the mother liquid 1.25 gm. benzoic acid were recovered. The yield of terephthalic acid was 12.65 gm. 7 Taking into consideration the amount of henzoic acid recovered, this corresponds to a yield of 56.9%

of theory.

Example X A homogeneous mixture of 30 gm. potassium benzoate,

13 gm. anhydrous potassium carbonate and 2 gm. cadmium fluoride was placed into an autoclave having a net volume of 0.2 liter. About 140 gm. liquid carbon dioxide were added thereto. Subsequently, the contents of the autoclave were heated for 30 hours at 300 C. whereby a maximum pressure of 670 atmospheres was reached. After treatment of the reaction product in the same manher as described in Example I, 13.20 gm. benzoic acid were recovered. The yield of terephthalic acid was 3.6 gm. Taking into consideration the amount of benzoic acid recovered, this corresponds to a yield of 60.1% of theory.

Example XI 40 gm. of a mixture consisting of sodium benzoate and sodium carbonate (molar ratio 1:05) and 2 gm. cadmium fluoride were placed into an autoclave having a net volume of 0.2 liter. About 150 gm. liquid carbon dioxide were added thereto. Subsequently, the contents of the autoclave were heated for 7 hours at 360 C. whereby a maximum pressure of 1400 atmospheres was reached. After further processing of the reaction mixture in accordance with the method described in Example I, 8.8 gm. benzoic acid were recovered. The yield of terephthalic acid was 4.15 gm. Taking into consideration the amount of benzoic acid recovered, this corresponds to a yield of 19.0% of theory.

Example XII For this experiment and the experiment described in Example XIII a high-pressure autoclave having a net volume of about 600 cc. was used. The autoclave was heated on all sides by an aluminum block. The temperature was controlled with the aid of eight platinum resist ance thermometers. Three of these were located in the aluminum block, three others in apertures in the wall of the autoclave, one in the bottom of the autoclave and another in a thermometer fitting submerged in the reaction mixture. The heating of the autoclave was controlled in such a way that no excess heat was developed in the wall of the autoclave. After the reaction temperature was reached, the temperature measured in the reaction mixture was lower by only 10 to 15 than the temperature in the wall of the autoclave;

132 gm. of an equimolar mixture of potassium benzoate and potassium carbonate was intimately admixed with 5.5 gm. cadmium fluoride in a ball mill, and the mixture was then placed into the high-pressure autoclave above described. After the autoclave was cooled to 0 C., 400 gm. of liquid carbon dioxide were added. Thereafter, the autoclave was heated for 10 hours to a wall temperature of 400 C., during which the internal pressure rose to approximately 1500 atmospheres. .The temperature measured in the reaction mixture'was about 390 C.

The reaction product, which weighed 148 gm., was worked up in the manner described above. 54.1 gm. terephthalic acid were obtained, corresponding to a yield of 73.8% of theory. 7.4 gm. benzoic acid were recovered from the mother liquor.

' Example XIII 131 gm. of an equimolar mixture of potassium benzoate and potassium carbonate were intimately admixed with 8 gm. of the complex salt K (CdF Cl in a ball mill, and the mixture was placed into the high-pressure autoclave described in Example XII. 400 gm. liquid carbon dioxide were added thereto. Thereafter the autoclave was heated for 10 hours to a wall temperature of 415 C. (internal temperature 400-405 C.) during which the internal pressure reached about 1500 atmospheres. The reaction prod- Example XIV A mixture of 16.1 gm. of the potassium salt of nicotinic acid (pyridine-fl-carboxylic acid), 138 gm. potassium carbonate and 1.0 gm.' cadmium fluoride was heated for 8 acid) crystallized out.

. temperature.

"water.

hours-at 350 C. in an autoclave having a capacity of 0.2

liter. At the beginning of the run, 180 gm. carbon dioxide were introduced into the autoclave. At 350 C. a pressure of 1800 atmospheres developed. After cooling and releasing the pressure from the autoclave, the reaction product, which weighed 32 gm., was dissolved in 400 cc. hot water. The solution was filtered, acidified with hydrochloric acid and then evaporated to one-half its volume. Upon cooling to C., 16.1 gm. of the monopotas sium salt of isocinchomeric acid (pyridine-2,5-dicarboxylic Example XV A mixture of 52.5 gm. of the potassium salt of B-naphthoic acid, 34.5 gm. potassium carbonate and 217 gm. cadrnium fluoride was heated for 6 hours at 420-430 C. in an'autoclave having a capacity of 0.6 liter. Prior to heating, 480 gm. carbon dioxide were introduced into the autoclave, which produced a pressure of 1350 atmospheres at the reaction temperature. After cooling and releasing the pressure from the autoclave, the reaction product was Example XVI A mixture of 22.0;gm. of the potassium salt of thiophene-a-carboxylic acid, 27.6 gm.- potassium carbonate and 2.0 gm. cadmium fluoride was heated in an autoclave for 3 /2 hours at 340 C. Before heating, the air was displaced with carbon dioxide and thereafter suflicient carbon dioxide was introduced into the autoclave to produce an internal pressure of 800 atmospheres at the reaction The reaction product, which weighed 54.2

gm., was dissolved in 600 cc. hot water. The solution 1 was filtered, and the filtrate was acidified with hydrochloric acid. The thiophene-Z,5'-dicarboxylic acid precipitated thereby was filtered off, washed with water and dried.

The yield was 17.0 gm. By extraction with ether, 2.2 gm. of amixture of thiophene-monocarboxylic and dicarboxylic acids having an acid number of 5 95 was recovered from the mother liquor and the wash water.

Example XVII dioxide, and then sulficient carbon dioxide was introduced under pressure to produce an internal pressure of 1500 atmospheres at the reaction temperature. The reaction product, which weighed 64.5 gm." was dissolved in hot The solution was filtered, and the clear filtrate was acidified with a quantity of hydrochloric acid equivalent to the calculated amount of potassium present. Upon cooling, 26.2 gm. of the monopotassium salt of trirnestinic acid (pyridine-2,4,6-tricarboxylic acid) crystallized out. By extraction of the mother liquor, 2.76 gm. additional pyridine-tricarboxylic acid were obtained.

Other alkali metal salts such as the lithium, rubidium and cesium salts or the alkaline earth metal or thallium salts may be employed in place of the potassium and sodium salts, with similar results. In all of the above examples, yields have been calculated on the assumption that 1 mol benzoic acid forms 1 mol terephthalic acid.

the following claims.

This application is acontinuation-in-part of our previous application Serial No. 643,952, filed March 5, 1957, now abandoned.

We claim:

1. In the method of producing alkali metal salts of aromatic carboxylic acids, selected from the group consisting of monoand dicyclic aromatic and aromatic heterocyclic diand tricarboxylic acids from the corresponding monoand dicyclic aromatic and aromatic heterocyclic mono-carboxylic acid salts which comprises, heating the mono-carboxylic salts to be converted to a temperature above 300 C. and below the temperature at which said salt and the reaction products substantially decompose in a substantially oxygen-free inert carbon dioxide atmosphere and in the presence of an acidbinding agent,-for a time sufficient to effect said conversion, the improvement which comprises conducting the reaction under a pressure of at least 400 atmospheres, thereby obtaining a molar yield of the diand tricarboxylic acid product salts which is more than 50% of the monocarboxylic acid salts undergoing conversion.

2. In the method of producing alkali metal salts of aromatic carboxylic acids, selected from the group consisting of monoand dicyclic aromatic and aromatic iron, in a substantially oxygen-free carbon dioxide atmosphere and in the presence of a metallic salt acidbinding agent, for a time sufficient to effect said conversion, the improvement which comprises conducting the reaction under a pressure of at least 400 atmospheres, thereby'obtaining a molar yield of the diand tricarboxylic acid product salts which is more than 50% of the monocarboxylic acid salts undergoing conversion.

3. In a process for the production of terephthalic acid from potassium benzoate which comprises the steps of heating said potassium benzoate at a temperature of at least 300 C. but not greater than the temperature at which said salts and the reaction products will substantially decompose, in the presence of a cadmium-containing catalyst and potassium carbonate in a carbon dioxide atmosphere, for a time suflicient to efiect said conversion, whereby said potassium benzoate undergoes a conversion to dipotassium terephthalate, and treating said terephthalate with and acid substance to liberate terephthalic acid, the improvement which comprises conducting the reaction under a pressure of at least 400 atmospheres, thereby obtaining a molar yield of terephthalic acid which is more than 50% based on the henzoate salts undergoing conversion.

4. The process of claim 3 wherein sodium benzoate is admixed with the reaction mixture.

5. In a process for the production of naphthalene- 2,6-dicarboxylic acid by the thermal conversion of the potassium salt of fi-naphthoic acid which comprises the steps of heating said potassium salt of S-naphthoic acid at a temperature of at least 300 C. but not greater than' the temperature at which said salts and the reaction products will substantially decompose, in the presence of a cadmium-containing catalyst and potassium carbonate ducting the reaction under a pressure of at least 400 at mospheres, thereby obtaining a molar yield of 2,6-dicarboxylic acid which is more than 50% based on the ,8- naphthoic acid salt undergoing conversion.

6. In a process for the production of isocinchomeric acid from potassium nicotinate, which comprises the steps of heating said potassium nicotinate at a temperature of at least 300 C. but not greater than the temperature at which said salts and the reaction products will substantially decompose, in the presence of a cadmium-containing catalyst and potassium carbonate in a carbon dioxide atmosphere, for a time sufiicient to eifect said conversion, whereby said potassium nicotinate undergoes a conversion to the dipotassium salt of isocinchomeric and treating said dipotassium salt of isocinchomeric with an acid substance to liberate isocinchomeric 'acid, the improvement which comprises conducting the reaction under a pressure of at least 400 atmospheres, thereby obtaining a product yield greater than 50% based on the amount of starting material.

7. In a process for the production of thiophene-2,5- dicarboxylic acid from the potassium salt of thiophene-acarboxylic acid, which comprises the steps of heating said potassium salt of thiophene-a-carboxylic acid at a temperature of at least 300 C. but not greater than the temperature at which said salts and the reaction products will substantially decompose, in the presence of a cadmiumcontaining catalyst and potassium carbonate in a carbon dioxide atmosphere, for a time suiiicient to etfect said conversion, whereby said potassium salt of thiophene-acarboxylic acid undergoes a conversion to the dipotassium salt of thiophene-Z,S-dicarboxylic acid and treating said dipotassium salt of thiophene-2,5-dicarboxylic acid with an acid substance to liberate thiophene-2,5-dicarboxylic acid, the improvement which comprises conducting the reaction under a pressure of at least 400 atmospheres,

thereby obtaining a product yield greater than based on the amount of starting material.

8. In a process for the production of pyridine-2,4,6- tricarboxy-lic acid from the potassium salt of isonicotinic acid, which comprises'the steps of heating said potassium salt of isonicotinic acid at a temperature of at least 300 C. but not greater than the temperature at which said salts and the reaction products will substantially decompose, in the presence of a cadmium-containing catalyst and potassium carbonate in a carbon dioxide atmosphere, for a time sufiicient to effect said conversion, whereby said potassium salt of isonicotinic acid undergoes a conversion to the potassium salt of pyridine 2,4,6-tricarboxylic acid and treating said potassium salt of pyridine 2,4,6-tricarboxylic acid with an acid substance to liberate pyridine- 2,4,6-tricarboxylic acid, the improvement which comprises conducting the reaction under a pressure of at least 400 atmospheres, thereby obtaining a product yield greater than 50% based on the amount of starting material.

References Cited in the file of this patent UNITED STATES PATENTS 2,794,830 Raecke et a1. June 4, 1957 2,823,229 Raecke et a1 Feb. 11, 1958 2,823,230 Raecke et al Feb. 11, 1958 2,823,231 Raecke et al Feb. 11, 1958 2,848,487 Keen Aug. 19, 1958 2,900,386 Raecke et al Aug. 18, 1959 2,906,774 Raecke et a1 Sept. 29, 1959 FOREIGN PATENTS 524,035 Belgium Mar. 2, 1956 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3,043,846 July 10, 1962 Bruno Blaser et a1a It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 73, for "360 C. read 340 C.

Signed and sealed this 13th day of November 1962.,

SEAL) \ttest:

IRNEIST w. SWIDER DAVID LADD Ittesting Officer Commissioner of Patents 

1. IN THE METHOD OF PRODUCING ALKALI METAL SALTS OF AROMATIC CARBOXYLIC ACIDS, SELECTED FROM THE GROUP CONSISTING OF MONO- AND DICYCLIC AROMATIC AND AROMATIC HETEROCYCLIC DI- AND TRICARBOXYLIC AICDS FROM THE CORRESPONDING MONO- AND DICYCLIC AROMATIC AND AROMATIC HETEROCYCLIC MONO-CARBOXYLIC ACID SALSTS WHICH COMPRISES, HEATING THE MONO-CARBOXYLIC SALTS TO BE CONVERTED TO A TEMPERAWTURE ABOVE 300* C. AND BELOW THE TEMPERATURE AT WHICH SAID SALT AND THE REACTION PRODUCTS SUBSTANTIALLY DECOMPOSE IN A SUBSTANTIALLY OXYGEN-FREE INERT CARBON DIOXIDE ATMOSPHERE AND IN THE PRESENCE OF AN ACIDBINDING AGENT, FOR A TIME SUFFICIENT TO EFFECT SAID CONVERSION, THE IMPROVEMENT WHICH COMPRISES CONDUCTING THE REACTION UNDER A PRESSURE OF AT LEAST 400 ATMOSPHERES, THEREBY OBTAINING A MOLAR YIELD OF THE DI- AND TRICARBOXYLIC ACID PRODUCT SALTS WHICH IS MORE THAN 50% OF THE MONOCARBOXYLIC ACID SALTS UNDERGOING CONVERSION. 